Method for depolymerizing polymethylmethacrylate

ABSTRACT

The present invention relates to a process of recovering monomeric esters of substituted or unsubstituted acrylic acid from polymer material having corresponding structural units by depolymerization by means of a fine-grained heat-transfer medium which is maintained above the depolymerization temperature of the polymer material. In a reactor, the polymer material is brought in contact with hot, mechanically fluidized heat-transfer medium. The resulting vapors are withdrawn and condensed, where the hot heat-transfer medium is continuously supplied at one end of the reactor, and cooled heat-transfer medium is discharged at the other end.

Process of Depolymerizing Polymethyl Methacrylate

This Application is a 371 of PCT/EP99/06852 filed on Sep. 16, 1999.

DESCRIPTION

The present invention relates to a process of recovering monomericesters of substituted or unsubstituted acrylic acids from polymermaterial having corresponding structural units.

Acrylate polymers, which include acrylic glasses chiefly consisting ofpolymethyl methacrylate (PMMA), are used for instance for the productionof long-lived consumer goods. For this purpose, there are frequentlyused molding processes in the course of which considerable amounts ofwaste polymer may be obtained. For expediently reprocessing theseproduction wastes and for utilizing waste materials recirculated fromthe process of utilization quite a number of proposals have now beenmade.

It is a well-known fact that acrylate polymers, above all PMMA, belongto the few plastic materials which are excellently suited for directchemical recycling. This means that at certain temperatures andpressures these polymers can completely be decomposed again into thecorresponding monomer units (depolymerization) when heat is supplied inan appropriate way (Grassie, N., Melville, H. W., Bull. Soc. Chim.Belges 1948, p. 142).

In the reports on the “19. Kunststofftechnisches Kolloquium desEurogress Aachen” (Mar. 11-13, 1998) there is described a continuouslyoperating process of depolymerizing PMMA. The comminuted plasticmaterial is charged into a hot extruder (ZSK 30), in which two tightlymeshing screws are rotating with a self-cleaning effect. By means ofthese screws, nondepolymerized PMMA and other residues are dischargedfrom the extruder. The PMMA will depolymerize in the extruder due to thethermal and the mechanical shearing effect. The resulting MMA iswithdrawn as vapor phase through the degassing bell and is condensed. Inthis process, the MMA content of the condensate varies between 89% and97%, the yield of MMA is about <97%. In the above-described process,heating the PMMA is effected in the extruder via the shell walls. Theratio of wall surface to reactor volume deteriorates, however, withincreasing plant. For large plants on an industrial scale the availableshell surface is so small that the extruder must either be madeextremely hot, in order to sufficiently decompose the PMMA, or only verymuch worse yields of MMA are obtained. The necessary increased heatingof the extruder shell, however, leads to a local overheating, whichcontributes to the formation of byproducts and impairs the monomerpurity.

Furthermore, it is known to depolymerize PMMA by means of afluidized-bed pyrolysis. As fluidized material there is used quartz sandhaving a grain size of 0.3 to 0.7 mm. It is a disadvantage of thisprocess that in the course of time the fluidized material is graphitizedwith soot. When the soot chips off the sand grain, it can be entrainedwith the gas stream. To obtain an appealingly clean monomer, this planttherefore requires the implementation of many special filter systems(cooler, cyclone, electrostatic precipitator). In this method, anitrogen stream is used for fluidizing the sand. It is likewisedisadvantageous that after the depolymerization the nitrogen and the MMAgas must again be separated by cooling. The nitrogen stream, which uponseparation from the product gas is recirculated to the reactor, musttherefore be cooled and heated in constant alternation, where thetemperature difference is at least 400 K. For a large-scale process,this is disadvantageous from an economical and ecological point of view(J. Franck, thesis 1993,Hamburg University).

It is the object of the invention to provide a process of recoveringmonomeric esters of substituted or unsubstituted acrylic acids frompolymer material having corresponding structural units, which allows acontinuous depolymerization free from residues, and thus provides forthe production of high-quality recycled monomeric esters in a highyield. In accordance with the invention, free from residues isunderstood to be a process which avoids the formation of deposits in thereactor space and thus makes it superfluous to shut down the plant forremoving the deposits, so that a continuous operation is ensured.

Furthermore, it is the object of the invention to provide a process asmentioned above, which can be operated on an industrial scale and helpsto eliminate the disadvantages such as a poor transfer of heat duringthe depolymerization, a high amount of apparatus required as well asenergetically unfavorable process flows.

The subject-matter of the invention is a process of depolymerizing PMMA,which is characterized in that in a reactor the polymer material isbrought in contact with a hot mechanically fluidized solid(heat-transfer medium), and the resulting vapors are withdrawn andcondensed, where the hot heat-transfer medium is continuously suppliedat one end of the reactor, and cooled heat-transfer medium is dischargedat the other end.

By means of the inventive process the reactor volume can be kept smallas a result of the very good heat transfer of the fine-grained solid andthe related relatively short dwell time of the polymer material. Hence,the dwell time of the resulting monomer vapors in the reactor is lessthan 6 seconds. The desired esters of the acrylic acids are obtained invery good yields and with a high purity. Thus, the hot, fine-grainedheat-transfer medium also ensures that in large-scale plants asufficient transfer of heat is ensured during the depolymerization.

In accordance with the invention, the mechanically fluidizedfine-grained heat-transfer medium produces a rubbing effect in thereactor, which helps to completely prevent an accretion of byproductsresulting from the depolymerization at the inner walls and installationsof the reactor. These depolymerization byproducts are continuouslydischarged from the reactor together with the fine-grained heat-transfermedium, so that an agglomeration of the byproducts in the reactor isprevented. Thus, it is possible to continuously perform the advantageousprocess, as no depolymerization residues, which otherwise must beremoved from time to time from corresponding plants of the prior art,are left in the reactor. The monomer gas stream, which leaves thereactor, has a sufficient purity and need only be liberated fromentrained dust particles by means of a cyclone. A separation from acarrier gas stream, which would even entrain more dust particles, is byno means necessary.

The mechanical fluidization and the transport of the fine-grainedheat-transfer medium can be achieved by all possibilities well-known tothe man skilled in the art, such as by moving or rotating walls,possibly under the influence of gravitation. There is preferred theembodiment where the substances supplied to the reactor are mechanicallyfluidized, mixed with each other and conveyed in a mixer by means of oneor more intermeshing shafts rotating in the same direction, which areprovided with coils or other mixing tools. Approximately the same dwelltime of all solid particles (plug flow) in the range from 5 to 60seconds can be adjusted by changing the rotational speed of the screws.

The polymer material is heated within a short period and depolymerizedby means of the hot heat-transfer medium fluidized by the coils ormixing tools. The volatile components are discharged, whereas the solidbyproducts remaining after the depolymerization are discharged from thereactor together with the heat-transfer medium, so that a contaminationof the withdrawn monomer vapors with components originating from thesolid byproducts is likewise very advantageously prevented. The massbalance of the heat-transfer medium in the reactor is preferablymaintained by supplementing at the top end from a heated receiver.

As indicated above, the transport of the heat-transfer medium in thereactor can preferably be effected by one or more rotating shafts, whichare equipped with coils or other mixing tools, from the inlet opening tothe outlet opening.

Having been discharged from the reactor, the heat-transfer medium can besupplied to the bottom end of a pneumatic conveying line via a secondarydegassing tank. The subsequent heating of the heat-transfer medium canbasically be effected with all methods known to the man skilled in theart. Preferably, a hot, possibly oxygenous gas stream and/or possiblyadditional fuels are supplied to the bottom end of the pneumaticconveying line via the combustion chamber. The resulting gas streamsupplies the heat-transfer medium to the top, where at the same timecombustible residues from the depolymerization and additional fuels areburnt. Accordingly, this leads to the reheating of the fine-grainedheat-transfer medium. The mixture of heat-transfer medium and gasesreaches a heat transfer separator, from which gases and fine dustparticles (e.g. color pigments contained in the PMMA) are withdrawn viaa dust separator (cyclone, exhaust gas filter). The heat-transfer mediumseparated in the heat transfer separator trickles downwards and reachesa collecting tank which serves as hot heat transfer receiver for thereactor.

The temperature of the heat transfer solid at the reactor inlet dependson the ratio of the mass flows heat-transfer medium/polymer material.With a ratio of 10:1 a superheating of the heat-transfer medium of 150°C. is obtained, with a ratio of 5:1 a superheating of 300° C. During thedepolymerization, the reaction mixing temperature of the heat-transfermedium can lie in the range between 300° C. and 650° C., preferablybetween 350° C. and 450° C. However, the heat-transfer medium heated bythe hot gas stream has a temperature of 400° C. to 900° C., preferably500° C. to 750° C.

As heat-transfer medium, every inorganic fine-grained solid (grain sizebetween 0.1 and 5 mm, preferably 0.3 and 2.0 mm) can be used, which hasthe required strength and a sufficient stability with respect totemperature changes and oxygen. In many cases, screened sand was useful,which according to DIN 4222 is called coarse sand. However, there mayalso be used other naturally occurring or synthesized oxides on thebasis of silicon, aluminium, magnesium, zirconium or also mixtures ofthese elements.

With the process in accordance with the invention, the dwell time of thevapors and gases formed in the reactor before the condenser can be lessthan 6 seconds, preferably less than 2 seconds. The dwell time of thefine-grained heat-transfer medium in the reactor is freely selectable.It is preferably in the range from 5 to 60 seconds.

The ratio of hot heat-transfer medium to PMMA in the reactor is likewisefreely selectable in wide ranges. What seems to be expedient andpreferred is a ratio between 3:1 and 30:1.

In accordance with a further aspect, the inventive process can improvethe economy of the recovery as well as the quality of the productobtained in that the vapors withdrawn are treated with condensate, whichwas cooled in a monomer circulating cooler, and in that the condensateresulting from the treatment is recirculated to the cooler, where it iscooled and partly recirculated for treating the vapors withdrawn, andthe remaining part of the condensate is discharged for furtherprocessing and product recovery.

The crude vaporous depolymerizate, i.e. the depolymerization gases, isfirst of all sprayed by means of a nozzle as in a shower with a part ofthe condensate previously cooled in the monomer circulating cooler andrecirculated. Due to the direct contact of the depolymerization gaseswith the atomized crude condensate liquid a fast intensive cooling and avery short dwell time of the vaporous depolymerizate at thedepolymerization temperature is achieved, which leads to a distinctimprovement of the monomer yield and quality. The vapors withdrawn arequenched with condensate in a concurrent process. Solid deposits, whichare produced on the otherwise usual coolers, can thereby be reducedconsiderably. Since the actual coolers only get in contact with adistinctly cooler, already condensed crude product, the risk ofaccretion or deposits is reduced. A reduction of the coating thicknesson coolers can, however, additionally contribute to an improved qualityof the crude condensate, in particular to an increased monomer content.

The condensate used for treating the monomer gases can be cooled in themonomer circulating cooler to a temperature between about 5 and 40° C.,preferably 20 and 30° C. With parts of this cooled condensate, thevapors withdrawn from the reactor are then expediently cooled duringquenching to a condensate temperature between 20 and 50° C., preferably35 and 50° C.

The non-condensable vapors and gases of the condensate are expedientlyintroduced into the riser, in which the fine-grained heat-transfermedium is heated, and also burnt.

In principle, the polymer material can be supplied to the reactor inevery conceivable form. The commonly used introduction devices weresuccessfully used in conjunction with conveyor belts, screws or thelike. Larger pieces such as plates or molded articles can easily becomminuted to the size required or desired for the reactor, for instanceby preceding shredders or mills. In the final analysis, the size of theprocessable polymer material pieces also depends on the properties ofthe polymerizates and the capacity of the reactor.

In general, the polymer material to be fed into the reactor and to bedepolymerized there may be present in any conveniently processable form,for instance as chips, as granules, as fine powder, as shavings or ascoarsely shredded material. These forms of addition may be introducedalone or in combination. Moreover, the more or less solid forms ofaddition can also be introduced together with liquid monomer in a moreor less pure or contaminated form. When performing the invention,granules with a preferred grain size of about 1 to 10 mm wereparticularly useful.

The polymers to be fed into the reactor in the process in accordancewith the invention chiefly contain structural units which in terms oftheir chemical structure satisfy the following formula I:

wherein

R¹ is C₁₋₆alkyl, preferably C₁₋₄alkyl,

R² is H, C₁₋₆alkyl, preferably H or C₁₋₄alkyl, with H or CH₃ beingparticularly preferred, and

n is a positive integer larger than 1.

Exemplary compounds include polymethyl acrylate, polyethyl acrylate,polymethyl methacrylate, polypropyl acrylate, polybutyl acrylate,polypropyl methacrylate, polybutyl methacrylate and copolymers whichhave two or more of these types of polymer. The first four compounds arepreferred within the scope of the invention. Particularly preferred ispolymethyl methacrylate (PMMA).

In addition to the chemical mixtures (random copolymers or also blockcopolymers), which were obtained by copolymerizing at least twosubstituted or unsubstituted acrylic-acid ester monomers (e.g.methylmethacrylate-n-butylmethacrylate copolymers), copolymers can beprocessed with the inventive process which have up to 50 wt-% of atleast one further vinylically unsaturated monomer which can becopolymerized with at least one substituted or unsubstitutedacrylic-acid ester monomer.

Typical examples include for instance methylmethacrylate-styrenecopolymers or methylmethacrylate-butylacrylate-styrene terpolymers.

Physical mixtures, so-called blends, can also be reprocessed inaccordance with the invention. As regards the reprocessing of polymermaterial in accordance with the inventive process, merely thefundamental possibility of the nondestructive depolymerization (withrespect to the monomers) as well as the possibility of separating theresulting vapor mixture in a fractionated distillation or with othercommonly used methods of separation must be seen as limiting factors.When depolymerization and separation are possible in principle, there isno fundamental obstacle to using the process in accordance with theinvention.

BRIEF DESCRIPTION OF THE DRAWING

The invention will subsequently be explained in detail with reference toFIG. 1 of the drawing.

FIG. 1 shows a recipient vessel 1 for the polymer material, which at thetop end is charged into a metering screw 2. With its end, the meteringscrew 2 opens into the top end of the reactor 3,which at this end alsohas a means for supplying hot heat-transfer medium from the receiver 11.At its end, the reactor 3 is on the one hand connected with a recipientvessel 9 (gas generator) for the discharged heat-transfer medium and onthe other hand with a cyclone 4. From the cyclone 4, a further supplyline opens into the condenser 5 which is connected with a monomer tank6. Subsequent to the monomer tank 6 a monomer pump 7 is provided, whichon the one hand supplies the condenser 5 via a monomer circulatingcooler 8 and on the other hand provides product A to be discharged.

The dust separated in the cyclone 4 is recirculated to the gas generator9, which in turn has a supply line to the riser 10. From the monomertank 6, non-condensable gases are likewise supplied to the riser 10 viaa blower 16, in which riser they are thermally disposed of (burnt).

Hot, oxygenous flue gas from the combustion chamber 15 and fuel fromline 17 are still charged into the riser 10. The combustion chamber 15is likewise supplied with fuel from line 17 and air from the blower 14.At the top, the riser 10 opens into the heat transfer separator 12,which has an outlet to a dust separator 13 (exhaust gas filter orcyclone) in which exhaust gas B and dust C are separated.

How the apparatus shown in FIG. 1 is operated can be taken from thefollowing Example.

EXAMPLE IN ACCORDANCE WITH THE INVENTION

Via the polymer recipient vessel 1 1000 kg/h PMMA waste granules aremetered into the reactor 3 via the metering screw 2. At the same time,10,000 kg/h sand heated to about 550° C. are metered from the recipientvessel 11 into the reactor 3, in which the mixing of the above flowsresults in a depolymerization temperature of 400° C.

In the depolymerization, 5 kg/h solid residues as well as 995 kg/h gasesand vapors are obtained, which are introduced into the condenser 5 via acyclone 4, largely liberated from dust. In said condenser, they arecharged with condensate, which was cooled to 25° C. in the monomercirculating cooler 8, and condensed before the condensate flows into themonomer tank 6. Via a pump 7, the monomer stream is pumped through thecirculating cooler 8 and in part reused for the condensation of monomervapors as well as guided to the discharge A in an amount of 990 kg/h.The 5 kg/h non-condensable gases obtained in the depolymerization aresucked off via the blower 16 and supplied to the riser 10, where theyare burnt.

The 10,000 kg/h sand from the gas generator 9 together with the 5 kg/hresidues from the depolymerization flow into the riser 10 with atemperature of 400° C. and by the hot gas formed in the combustionchamber 15 are pneumatically delivered into the heat transfer separator12 via the riser 10 and reheated to 550° C. At the bottom end of theriser 10 additional fuel, e.g. heating oil, is added via line 17. Bymeans of the excess atmospheric oxygen from the combustion chamber 15,the additional fuel as well as the organic depolymerization residue areburnt or oxidized. In the recipient vessel 11, the inorganic pigmentsand the heat transfer dust are separated from the coarser-grained sandby screening. The same is obtained in the receiver 11, whereas the fluegas and the fine dust get into the dust separator 13, where gas B isseparated from dust C. The amount of MMA recovered from the condensate Ais 958 kg/h (95.8% yield).

List of Reference Numerals:

No. Designation A MMA product stream B exhaust gas C dust 1 PMMArecipient vessel 2 metering screw 3 LR mixer-reactor 4 cyclone 5condenser 6 monomer tank 7 monomer circulating pump 8 monomercirculating cooler 9 gas producer 10 riser 11 heat transfer receiver 12heat transfer separator 13 dust separator 14 burner air blower 15combustion chamber 16 exhaust gas blower 17 fuel line

The reactor 3 of the Example is a LR mixer-reactor which is known perse, e.g. from “Erdoel und Kohle-Erdgas-Petrochemie/HydrocarbonTechnology” No. 42 (1989), pages 235-237. The reactor employsintermeshing shafts rotating in the same direction.

What is claimed is:
 1. A process for continuously recovering a monomericester of a substituted or unsubstituted acrylic acid free from adepolymerization residue from a polymer material having correspondingstructural units, which comprises the steps of: (a) continuouslysupplying a hot heat transfer medium heated to a temperature between 400and 900° C. to an inlet of a fluidizing reactor containing an inner walland intermeshing rotating shafts which are provided with mixing tools;(b) adding the polymer material having corresponding structural units toan inlet in the fluidizing reactor; (c) depolymerizing in the fluidizingreactor at a temperature of 300 to 650° C. the polymer material with thehot heat transfer medium where the polymer material and the hot heattransfer medium are mechanically fluidized by intermeshing rotatingshafts which are provided with mixing tools for rubbing the polymermaterial to prevent an accretion of a residue on the inner wall of thereactor, said residue resulting from the depolymerization of thepolymeric material thereby preventing an agglomeration of thedepolymerization residue in the reactor to obtain vapors of themonomeric ester of a substituted or unsubstituted acrylic acid free fromthe depolymerization residue and cooled heat transfer medium containingthe depolymerization residue wherein the vapors of the monomeric esterhave a dwell time in the fluidizing reactor of less than 6 seconds; (d)continuously separately discharging from outlets in the fluidizingreactor the vapors of the monomeric ester of a substituted orunsubstituted acrylic acid free from the depolymerization residue andthe cooled heat transfer medium containing the depolymerization residue;and (e) condensing the vapors of the monomeric ester of a substituted orunsubstituted acrylic acid free from the depolymerization residue toobtain a condensate free from the depolymerization residue and anon-condensable gas.
 2. The process defined in claim 1 wherein accordingto step (a) the hot heat transfer medium has a grain size between 0.1and 5 mm.
 3. The process defined in claim 1 wherein according to step(a) the hot heat transfer medium is an oxide of silicon, aluminum,magnesium, zirconium, or mixtures thereof.
 4. The process defined inclaim 1 wherein according to step (c) the hot heat transfer medium inthe fluidizing reactor has a dwell time that is freely selectable in therange of 5 to 60 seconds.
 5. The process defined in claim 1 whereinaccording to step (c) the ratio of the hot heat transfer medium and thepolymer material in the fluidizing reactor is freely selectable in therange of 3:1 to 30:1.
 6. The process defined in claim 1 whereinaccording to step (e) the vapors of the monomeric ester of a substitutedor unsubstituted acrylic acid free from the depolymerization residue aretreated with condensate free from the depolymerization residue which wascooled in a monomer circulating cooler, and the condensate resultingfrom the condensation treatment is introduced into the monomercirculating cooler where the condensate is further cooled and in partrecirculated for condensing additional quantities of the vapors of themonomeric ester of a substituted or unsubstituted acrylic acid free fromthe depolymerization residue, and the remaining part of the condensateis discharged for further processing and product recovery.
 7. Theprocess defined in claim 6 wherein the vapors are quenched withcondensate in a concurrent process.
 8. The process defined in claim 6wherein the condensate used for treating the vapors of the monomericester is cooled in the monomer circulating cooler to a temperaturebetween about 5° and 40° C.
 9. The process defined in claim 1 whereinfollowing step (e) the non-condensable gas is channeled to a riserleading to a heat transfer separator and the non-condensable gas isburnt in said riser to provide heat for heating additional hot heattransfer medium supplied during step (a).
 10. A process for continuouslyrecovering a monomeric ester of a substituted or unsubstituted acrylicacid free from a depolymerization residue from a polymer material havingcorresponding structural units, which comprises the steps of: (a)continuously supplying a hot heat transfer medium heated to atemperature between 400 and 900° C. to an inlet of a fluidizing reactorcontaining an inner wall and intermeshing rotating shafts provided withmixing tools; (b) adding the polymer material having correspondingstructural units to an inlet in the fluidizing reactor; (c)depolymerizing in the fluidizing reactor at a temperature of 300 to 650°C. the polymer material with the hot heat transfer medium where thepolymer material and the hot heat transfer medium are mechanicallyfluidized by the intermeshing rotating shafts provided with mixing toolsfor rubbing the polymer material to prevent an accretion of a residue onthe inner wall of the reactor, said residue resulting from thedepolymerization of the polymeric material thereby preventing anagglomeration of the depolymerization residue in the reactor to obtainvapors of the monomeric ester of a substituted or unsubstituted acrylicacid free from the depolymerization residue and cooled heat transfermedium containing the depolymerization residue wherein the vapors of themonomeric ester have a dwell time in the fluidizing reactor of less than6 seconds; (d) continuously separately discharging from outlets in thefluidizing reactor the vapors of the monomeric ester of a substituted orunsubstituted acrylic acid free from the depolymerization residue andthe cooled heat transfer medium containing the depolymerization residue;(e) liberating the depolymerization residue from the cooled heattransfer medium by reheating the cooled heat transfer medium to atemperature between 400° C. and 900° C. by means of a hot gas stream,said hot gas stream moving the reheated heat transfer medium into a heattransfer receiver communicating with the fluidizing reactor; (f)recycling the reheated heat transfer medium from the heat transferreceiver to the fluidizing reactor according to step (a); and (g)condensing the vapors of the monomeric ester of a substituted orunsubstituted acrylic acid free from the depolymerization residue toobtain a condensate free from depolymerization residue and anon-condensable gas.